1. Field of the Invention
The present invention relates to a low power fan drive circuit to increase the overall efficiency of a power supply in a personal computer system.
2. Description of the Prior Art
A personal computer system typically includes a fan and a fan drive circuit to reduce heat build-up within the chassis of the computer system. The internal ambient temperature of the computer system is allowed to vary between roughly 0.degree. and 70.degree. C. The fan is preferably a variable speed fan having a rotational velocity or speed proportional to the voltage applied across the terminals of the fan. The fan drive circuit senses the ambient temperature within the computer and drives the fan to the appropriate speed based on the ambient temperature. In general, the fan preferably operates at an initial minimum speed at room temperature, with its speed increased if and when the temperature rises.
A variable fan suitable for use in cooling a personal computer system typically requires a voltage ranging between 5 and 15 volts for satisfactory operation. Typical computer systems require a plurality of levels of voltages to operate the circuitry, including high voltage outputs including a positive 12 volt supply, several intermediate output voltages including a positive 5 volt supply, and lower output voltages including a negative 5 volt supply. It has been considered convenient, therefore, to connect the fan between the positive 12 volt supply (+12 V signal) and the negative 5 volt supply (-5 V signal) to achieve the full voltage range for the fan. The +12 V signal was connected to the positive fan terminal. A thermistor circuit was typically provided between the negative fan terminal and the -5 V signal to draw the appropriate amount of current through the fan based on the ambient temperature of the power supply. The thermistor circuit preferably included a thermistor and an emitter-follower transistor circuit, so that the emitter follower transistor was biased by the thermistor to draw more current through the fan when the thermistor sensed an increase in ambient temperature.
A bleeder resistor was provided between the negative terminal of the fan and the -5 V signal to sense the voltage of the fan. A shut down circuit was connected to the bleeder resistor, which operated to shut down the power supply when the negative terminal of the fan dropped below a predetermined minimum voltage level, indicating an inoperative fan or an excessive temperature. The bleeder resistor was very inefficient since a significant current always flowed through it during all operating modes. The current through the bleeder resistor generated heat, causing a loss of energy and efficiency. In fact, upon startup and during normal operation at room-temperature, the voltage across the bleeder resistor was at a maximum, causing the greatest heat and inefficiency. Nonetheless, the bleeder resistor was considered necessary to sense abnormal conditions or high temperature. Thus, the bleeder resistor and its associated inefficiency was tolerated.
Recently, the U.S. Government has promulgated guidelines pursuant to its energy conservation program. Since these guidelines are relatively progressive and difficult to achieve, designers are seeking a plurality of ways to reduce inefficiencies of prior art designs. Therefore, it is considered desirable to improve the efficiency of the fan drive circuit without significantly improving cost while maintaining safe operation.